Many communications buses and controllers have been developed using a number of communications modes on either parallel or serial buses depending on suitability for given applications.
Isochronous or Circuit Switched Services and Packet or Framed services are already widely used and Asynchronous Transfer Mode (ATM) or Cell services are being introduced. These three communications modes are briefly characterised as follows.
Isochronous Services require guaranteed synchronous access to the bearer--e.g. receipt or delivery of an octet at precisely 125 .mu.secs. Their most typical usage is to convey voice information at 64 Kb/s, however they are often used for data services to take advantage of the flexible and cost-effective switching and routing capabilities incorporated in the globally connected telephony networks. Historically these services were very prone to errors and thus voice services were typically noisy and data often unreliable. More recently network bearers have become much less error prone.
Cells are employed in the so called Asynchronous Transfer Mode (ATM) technology embodied in CCITT's B-ISDN, Bell Corp's Switched Multi-Megabit Data Service (SMDS), and IEEE 802.6 Metropolitan Area Network (MAN) standard. Cells are comprised of fixed length packets or frames, 53 octets long, of which 48 octets are user data created by a source user and then rapidly routed on the fly through ATM switches to their destination. To assure that acceptable end user service delay criteria are met, cells require either Deterministic Guaranteed Access to match some maximum value, or Statistical Access with a tolerable Quality of Service, or some combination of these.
Packet Services are by comparison with Isochronous and Cell services very delay tolerant, and typically an end user's real time usage of such services is very peaky. By serially multiplexing the needs of many users a more consistent level of traffic is usually supported over a fixed bandwidth channel. Thus a Packet bearer service must offer a Multi-User (and therefore arbitrated) statistically queued (and therefore delayed) Service. Packet services grew out of a need for more reliable transfer of data in spite of poor lines. Information integrity remains a major requirement of Packet services.
These three services can be ranked in terms of their access priority demands as: Isochronous, Cell and Packet. Thus Isochronous must take precedence over Cell, and Cell over Packet.